1. Field of the Invention
The present invention relates to an optical apparatus such as a photosensor, a semiconductor laser, an optical amplifier, or the like, using a wavelength selective photocoupler which is constituted by coupling two waveguides through a diffraction grating.
2. Related Background Art
A conventional wavelength selective photocoupler is constituted by two optical waveguides formed on a single substrate, as described in, e.g., Applied Physics Letters, R. C. Alferness, et al. 33, p. 161 (1978), Japanese Patent Laid-Open No. 61-250607, or Study Reports OQE81-129, the Institute of Electronics and Communications Engineers of Japan. FIG. 1 is a schematic perspective view showing a structure of a conventional wavelength selective photocoupler.
In FIG. 1, two optical waveguides 81 and 82 are formed to have different line widths, heights, and the like, and hence, have different propagation constants of guided light beams propagating along the corresponding optical waveguides. In this case, a diffraction grating 83 for compensating for a difference between the propagation constants is formed on one of regions where electric field distributions of the two guided light beams are present, so that photocoupling between the two waveguides occurs with respect to a guided light component in a specific wavelength range. More specifically, only a light component in a specific wavelength range is selected to shift a light power between the waveguides.
Such a conventional photocoupler is applied to a wavelength selective filter for combining/dividing signal light and a light wave of a specific wavelength, and a photosensor for receiving a light wave of a specific wavelength.
However, in the conventional photocoupler, since the waveguides are formed on a single plane, a difference between the propagation constants of the two waveguides can only be controlled by line widths, heights, and the like of the waveguides, and a large propagation constant difference cannot be obtained. For this reason, an optical power shift operation between the two waveguides is caused not only by the diffraction grating but also by an interference effect of two guided light beams. Therefore, it is difficult to obtain sharp wavelength selectivity.
In the photocoupler described above, a direction of causing a power shift corresponds not to a direction of strongly confining guided light, i.e., a direction perpendicular to the substrate but to a planar direction with a relatively loose confinement effect. Therefore, a coupling length of a coupling region must be set as large as 3 to 15 mm. This coupling length makes the entire element large in size, and disturbs the manufacture of an element utilizing waveguides formed of a material which cannot neglect absorption.
In order to further change the width and height of the waveguide, a photolithographic technique is required. However, it is very difficult to attain precision of 1 .mu.m or less in the manufacture, resulting in poor reproducibility of elements and impairing element characteristics.
A wavelength selective photocoupler which can solve the above problems is proposed in Integrated and Guided-Wave Optics, R.C. Alferness et al., 1989, technical digest series vol. 4, pp. 215-218. In this photocoupler, two waveguide layers of different guided modes are stacked on a substrate, and are optically coupled through a diffraction grating.